The present invention relates to determination of state-of-charge of a battery.
The power demands for operating elevators range from positive, in which externally generated power (such as from a power utility) is used, to negative, in which the load in the elevator drives the motor so it produces electricity as a generator. The use of the motor to produce electricity as a generator is commonly called regeneration. In conventional systems, if the regenerated energy is not provided to another component of the elevator system or returned to the utility grid, it is dissipated through a dynamic brake resistor or other load. In this configuration, all demand remains on the power utility to supply power to the elevator system, even during peak power conditions (e.g., when more than one motor starts simultaneously or during periods of high demand). Thus, components of the elevator system that deliver power from the power utility need to be sized to accommodate peak power demand, which may by more costly and require more space. Also, the regenerated energy that is dissipated is not used, thereby decreasing the efficiency of the power system.
In addition, an elevator drive system is typically designed to operate over a specific input voltage range from a power supply. The components of the drive have voltage and current ratings that allow the drive to continuously operate while the power supply remains within the designated input voltage range. In conventional systems, when the utility voltage sags, the elevator system faults. In conventional systems, when a utility power failure occurs or under poor power quality conditions, the elevator may become stalled between floors in the elevator hoistway until the power supply returns to normal operation.
Elevator drive systems may incorporate a secondary power supply that is controlled to deliver supplemental power to the elevator hoist motor during periods of positive power demand, and store power from the power utility and/or the elevator hoist motor during periods of zero or negative power demand. For example, U.S. Pat. No. 6,431,323, Tajima et al., describes an elevator drive system including an energy storage apparatus and a controller for controlling charging and discharging operation of the energy storage apparatus based on a charging target value (e.g., a charge value based on the time of day). However, this type of control does not provide a direct method for gauging future energy demands of the elevator drive system, and does not control the upper and lower charge limits of the energy storage apparatus.
Elevators equipped with regenerative drives provide the possibility of recovering a significant portion of the energy employed to move the load and counterweight. The energy recovered as such may be sent back to the building grid or stored locally for future use by the elevators or other needs in the building which houses the elevators. Utilizing the stored energy to power the elevators is of particular interest to customers because various benefits and functionalities may then be realized, which include feeder size reduction enabled by the boost from the storage devices, and rescue operation resulting from an energy source secondary to the grid. It is essential to control the state of charge (SOC) of the battery to ensure the operability of the system, preserve battery life, and guarantee safe operation.
The battery SOC estimation methods reported in the prior art are generally based on an imprecise correlation between the SOC and measurable parameters such as battery module (or battery pack) voltage, current, and temperature. The complexity of the processes involved in battery operation makes the SOC estimate prone to error. Coulomb counting, based upon measurements by current sensors, is usually combined with a Kalman filter to estimate the state of the system. However, systematic errors that are not random may lead to cumulative error that is unlikely to be bounded. As a result, the state-of-the-art SOC estimators are capable of estimating the SOC with no better accuracy than ±15% in terms of the absolute value of the capacity of a battery. Consequently, the battery may operate out of the desirable SOC regime, which can potentially reduce the lifetime of the battery and degrade the energy efficiency of the battery. To avoid operation out of the desirable SOC regime it is advantageous to calibrate the SOC of the battery regularly, and thereby reset any SOC estimate and bound the error in that estimate.
Battery SOC calibration techniques are critical to the mobile communication and hybrid electrical vehicle industries. U.S. Pat. No. 6,630,814 by Ptasinski et al., aimed at calibrating the battery of mobile phones, teaches a coulomb counting method to regularly identify throughout the lifetime of a battery the currently available capacity. The method relies on fully charging and discharging the battery. U.S. Pat. No. 6,630,814 also teaches an alternative capacity estimation method based upon an aging trend of the battery when complete charging and discharging are not allowed. There are several drawbacks to the complete charging and discharging method. First, it requires a long time to complete, which may not be feasible because of the continuous operation of the battery. Second, fully charging and discharging the battery (representing 100% depth of discharge (DOD)) is likely to accelerate the degradation of the battery, resulting in significantly shorter battery life. Because the duty cycle of an elevator is often higher than that of average mobile phones, and the lifetime of the battery in an elevator system needs to be much longer, the methods similar to those disclosed by U.S. Pat. No. 6,630,814 may not be applicable to elevators.
U.S. Pat. No. 6,841,972 by Koo teaches a method of qualitatively resetting the SOC of the battery in hybrid electric vehicles based on the relationship between SOC and battery parameters. This method does not require fully charging and discharging the battery, and it can be implemented on-line. The SOC of a battery is divided into 15% to 25% brackets. The actual SOC of the battery is qualitatively estimated and assigned to one of the brackets.
Other patents and published applications discussing determination of battery state-of-charge include: U.S. Pat. No. 6,356,083 by Ying; U.S. Pat. No. 6,359,419 by Verbrugge et al.; U.S. Pat. No. 6,441,586 by Tate, Jr. et al.; U.S. Pat. No. 6,639,385 by Verbrugge et al.; U.S. Pat. No. 6,653,817 by Tate, Jr. et al.; U.S. Pat. No. 6,686,724 by Coates et al.; U.S. Pat. No. 6,927,554 by Tate, Jr. et al.; U.S. Pat. No. 7,375,497 by Melichar; U.S. Publication No. 2003/0214303 by Ying; U.S. Publication No. 2004/0162683 by Verbrugge et al.; U.S. Publication No. 2005/0189918 by Weisgerber et al.; U.S. Publication No. 2005/0231165 by Melichar; U.S. Publication No. 2006/0091861 by Melichar; U.S. Publication No. 2006/0091862 by Melichar; U.S. Publication No. 2006/0091863 by Melichar; U.S. Publication No. 2007/0159137 by Verbrugge et al.; U.S. Publication No. 2007/0285061 by Zettel et al.; U.S. Publication No. 2007/0285097 by Zettel et al.; and U.S. Publication No. 2008/0164849 by Ciaramitaro.
Various patents and published patent applications on SOC estimation do not adequately provide techniques of calibrating and resetting the battery on a quantitative basis. Additionally, the implementation of those calibration methods may have negative impact on the battery lifetime; hence, battery calibration methods require careful evaluation.